Public doubts over safety, permanence and accountability of Carbon Capture and Storage (CCS) have already slowed down its introduction. For example, the United Nations Framework Convention on Climate change in 2006 in Nairobi decided not to sanction geological storage because of political uncertainties. CCS research will have to focus on verification and monitoring to gain the trust of the public at large.
The public will not take on good faith that carbon dioxide (CO2) is safely and permanently stored in geologic reservoirs. Positive proof will be required. Ideally, measurements should provide a complete inventory at any time without having to rely on past measurements. Injection protocols from decades earlier, together with a promise that leakage would not have escaped observation, will be insufficient proof that a specific amount of CO2 remains stored in an underground reservoir.
As in real estate, verifiable monitoring and accounting schemes must be developed for geological storage of CO2. This requires tools for accurate inventory accounting and verification of the amounts of CO2 stored in a reservoir. These tools need to ensure that the amount of CO2 injected is equal to the amount claimed, and that losses during the injection stage and subsequent losses from storage are accurately determined.
However, injection measurements are far easier than accurate inventories of the CO2 that remains stored in the reservoir. Methods that can create such an inventory without having to rely on a historic record of injections and a continuous observation of potential leak paths would be highly preferable. There are a number of dynamic effects that make an accurate accounting of the CO2 difficult. For example, it is possible that a fraction of the CO2 migrates away from the storage reservoir. It may be that the leakage was detected, or it may be that no leakage was detected, since the relatively high background levels of CO2 present in the atmosphere and soil, coupled with seasonal fluctuations in CO2 fluxes, makes an accurate detection of slow leaks difficult. Chemical conversion and dissolution of CO2 open different transport routes and further complicate a full accounting.
Geophysical methods for detecting CO2, in situ are very powerful, but they are qualitative to semi-quantitative. Four-dimensional seismic, crosswell seismic, vertical seismic profiling (VSP), and wireline logging are excellent tools for tracking the migration of CO2 within a reservoir and providing certain information on CO2 concentration and saturation in case of VSP. In addition, several studies have demonstrated that under favorable conditions accumulations on the order of a few thousand tons of CO2 can be detected with seismic monitoring at a depth of one kilometer. Concerns will arise with leakage paths through regions with less favorable conditions and small local accumulations.
Most geophysical detection requires that CO2 is present as supercritical gas and cannot detect geochemical transformations into carbonates, or the dissolution of CO2 into brine. Therefore, they fail in establishing an accurate mass balance. Furthermore, formations that take on CO2 may already contain carbon that was resident in the formation before injection started or that moved into the formation after injection. For example, dissolution of limestone can add additional carbonate ions to the fluid. Depending on the site, the volumes of CO2 involved in these transitions can be very large and thus cannot be ignored. Also, excess pressure in the reservoir will result in changes in the surrounding formation. Even if these changes do not involve CO2 migration, they may be visible in 4D seismic and thus can create false positive signals of leakage. The lack of a signal does not prove the absence of leakage, nor is the presence of a signal sufficient to prove leakage.